The present invention relates to an electron beam lithography method, particularly to an electron beam lithography method capable of accurately detecting alignment marks while moving a wafer.
In a general electron beam lithography apparatus, a semiconductor wafer is set in a cassette (jig) mounted on a wafer stage to lithograph a chip pattern on the wafer.
The position of the pattern is relatively determined on the basis of alignment marks on the wafer.
An electron beam is deflected in accordance with a lithograph pattern. Therefore, when the position (coordinate values) of an alignment mark is deviated from a design value, it is necessary to correct the deflection signal of the electron beam by the deviation.
The position of an alignment mark is measured by controlling the wafer stage and setting the alignment mark on a wafer to a positional origin of the electron beam (which means the position of the electron beam in the case when the electron beam is not deflected by a deflector).
The alignment mark on the wafer is constructed with a wafer mark 11 which aligns the wafer 206 and a chip mark 12 which aligns chip patterns 13. The positional origin of the electron beam is usually measured by detecting the wafer mark which is moved by moving the wafer stage. Therefore, if the positions of the wafer mark 11 and the chip mark 12 are designed correctly without any positional design error, the positional origin of the electron beam measured by detecting the chip mark is measured correctly too.
But, when the wafer is deformed, the wafer mark is not set correctly on the positional origin of the electron beam measured by detecting the chip mark.
Moreover, positions of alignment marks are conventionally detected by applying the electron beam to the alignment marks and detecting electrons reflected from the marks by means of two reflected electron detectors set at positions symmetrical with respect to the electron beam origin. Therefore, if alignment marks are detected on a position which is apart from the positional origin of the electron beam or the reflected electron detectors are not disposed at positions symmetrical with respect to the origin, waveforms outputted by the reflected electron detectors become asymmetrical and thereby, errors occur in detecting positions of marks.
Japanese Patent Laid-open No. 57-211733(1982) discloses that alignment marks are scanned twice in two different directions and obtained mark detection signals are arithmetically averaged to decrease the errors due to the asymmetrical waveforms.
It is also performed to improve S/N of detection signals by scanning marks several times and arithmetically averaging obtained scanning signals to decrease the influence of random noises.
However, the above method has a problem that it takes much time and the apparatus throughput decreases because a plurality of marks on a wafer are successively measured by repeating the movement and stop of a stage (step-and-repeat system).
Therefore, Japanese Patent Laid-open No. 58200536(1983) discloses the so-called continuous stage moving system for detecting positions of marks while continuously moving a stage.
However, the continuous stage moving system has a problem that measurement errors increase due to asymmetry of detection signal waveforms because each mark position cannot be detected just under the positional origin of the electron beam though the mark detection time can be decreased.
Japanese Patent Laid-open No. 62-29136(1987) discloses a method for setting a function to estimate the skewness of detection signal waveforms and thereby correcting the asymmetry in accordance with scanning position.
Actually, however, asymmetry is not completely corrected because the skewness of mark detection signals cannot accurately be estimated.